Conventionally, when a laser beam is incident on a nonlinear optical crystal, the laser beam in which a wavelength has been converted by a nonlinear optical effect is emitted. In the case where a difference between a phase of a laser beam generated at a reference position and a phase of a laser beam generated at a position separated from the reference position, by a coherent length becomes π by phase conversion by a nonlinear optical crystal, the laser beams are mutually offset, and intensity thereof is decreased. A quasi phase matching (QPM) technique using a nonlinear optical crystal having a periodic polarization inversion structure is known to suppress the decrease in laser beam intensity. By this technique, a polarization inversion period is set so that a length in a light propagation direction in a positive or negative polarization region becomes a coherence length (length in which the phase is deviated by π).
Patent Literature 1 discloses a laser device which generates a laser beam by combining a nonlinear optical crystal and a semiconductor laser element, which does not resonate in an element thereof, and entirely resonating them.